Processes That Reduce Runoff Reservoirs

Impoundment of water in reservoirs (R) constitutes a significant sequestration of freshwater on land that would otherwise flow to the world's oceans. (See Fig. 5.3, color plate.) Hence it represents a major reduction in potential SLR. The world's largest reservoirs (>1 million m3) provide a maximum storage capacity of 4821 km.3 The total comes to 5078 km3, if other types of dams are included (e.g., concrete dams >15 m and concrete face rockfill dams), which contribute an additional 39 and 218 km3 respectively (Intl. Water Power & Dam Constr. Yearbook, 1998). (The dam data base was checked to eliminate redundancies.) Shiklomanov (1997), on the other hand, estimates a maximum reservoir capacity of 6000 km3. Small impoundments (<100 million m3) could add another 4.5% (L'vovich and White, 1990, p. 239), bringing the total global reservoir storage capacity to between 5306 and 6270 km3. These figures represent upper bounds, inasmuch as most dams are not filled to capacity. A reasonable assumption is that reservoirs are filled to 85% of capacity, on average. Thus, the total global volume in reservoirs lies between 4510 and 5330 km3. Since more than 90% of the total reservoir capacity has been created since the 1950s, and since reservoir capacity has grown at a linear rate (Chao, 1995; Shiklomanov, 1997), the volume of water impounded in reservoirs could potentially reduce SLR by an average of 0.27 to 0.33 mm/yr, at present (Fig. 5.4, Table 5.4).

A major dam-building boom is under way, especially in the developing world. A conservative estimate of the additional reservoir volume to be created by dams under construction is around 953 km3, or 810 km3 if 85% full. This latter volume represents 15 to 18% of present reservoir storage. It is equivalent to a potential withholding of another 2.3 mm of SLR. Evaporation from Reservoirs

Lakes, both natural and artificial, modify the climates of their surrounding areas, due to sharp land-water contrasts in temperature, albedo, and surface

Figure 5.4 Cumulative mass of water held in reservoirs since 1950, in billions of metric tons and in SLR equivalent (after Chao, 1995).

roughness and also differences in the moisture budget of these two surface types. Evaporation over lakes or reservoirs is greater than that over land, particularly if the surrounding terrain is arid or if reservoir temperatures exceed those of the overlying air. Evaporation from reservoirs constitutes an additional continental store of water. Although the bulk of the evaporated water probably reprecipitates regionally, a small fraction may be held as vapor in the atmosphere, particularly in drier regions. This fraction may be reduced to some extent by long-range transport of atmospheric moisture to the sea.

Evaporative losses from reservoirs are calculated as the difference in mean evaporation between the lake surface and that of dry land, using mean evaporation data for each climate zone. These losses, amounting to around 6.5 km3/ yr in 1950, increased to 164 km3/yr in 1990 and 188 km3/yr in 1995 (Shikloma-nov, 1997). Although the fraction of the evaporated water that is stored in the atmosphere is uncertain, a plausible upper bound lies between 1 and 2% (see Section 5.3.2 on irrigation, below). This increased atmospheric moisture could reduce SLR by 0.005 to 0.010 mm/yr (Table 5.4).

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